1. Field of The Invention
This invention relates to wireless communications and, more particularly, to a access control system for a cellular communications system.
2. Description of Related Art
FIG. 1 depicts a diagram of a wireless communications system 10 which provide wireless communications service to a number of wireless units (e.g., wireless units 12a-c) that are situated within a geographic region. The wireless cellular communications system 10 comprises a number of base stations 14a-e, geographically distributed to support transmission and receipt of communication signals to and from the wireless units 12a-c, which can be mobile or fixed, in the geographic region. Typically, each cell contains a base station, which comprises the radios and antennas that the base station uses to communicate with the wireless units in that cell. Each base station 14a-e handles voice and/or data communications over a particular region called a cell, and the overall coverage area for the cellular system is defined by the union of cells for all of the cell sites, where the coverage areas for nearby cell sites overlap to some degree to ensure (if possible) contiguous communications coverage within the outer boundaries of the systems coverage area. As depicted in FIG. 1, each cell is schematically represented by one hexagon in a honeycomb pattern; in practice, however, each cell has an irregular shape that depends on the topography of the terrain surrounding the cell. One cell site may sometimes provide coverage for several sectors. In this specification, cells and sectors are referred to interchangeably.
The base stations also comprise the transmission equipment that the base station uses to communicate with a mobile switching center (MSC) for the geographic region. An MSC 16 is responsible for, among other things, establishing and maintaining calls between the wireless units and calls between a wireless unit and a wireline unit (e.g., wireline unit 18). As such, the MSC interconnects the wireless units within the geographic region with a public switched telephone network (PSTN) 20. Within a geographic region, the MSC switches calls between base stations in real time as the wireless unit moves between cells, referred to as call handoff. The MSC 16 is connected to or integrated with a home location register (HLR) 22. The HLR 22 contains subscriber information and location information for all wireless units which reside in the geographic region of the MSC 16.
In a wireless cellular communications system, a base station and a wireless unit communicate voice and/or data over a forward link and a reverse link, wherein the forward link carries communication signals from the base station to the wireless unit and the reverse link carries communication signals from the wireless unit to the base station. There are many different schemes for determining how wireless units and base stations communicate in a cellular communications system. For example, wireless communications links between the wireless units and the base stations can be defined according to different radio protocols, including TDMA (time-division multiple access), FDMA (frequency-division multiple access), and CDMA (code-division multiple access).
In the context of wireless communications systems, subscriber access control allows a service provider to control the availability of communications links to wireless units. In the context of current CDMA systems, a purpose behind subscriber access control was to ensure the availability of communications links if an emergency overload condition should develop. As an example, a technical industry standard that is desirably observed in introducing access controls in a mobile communications system is TIA/EIA-95-A Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System (March, 1995) (xe2x80x9cIS-95-Axe2x80x9d); and ANSI J-STD-008 Personal Station-Base Station Compatibility Requirements for 1.8 to 2.0 GHz Code Division Multiple Access (CDMA) Personal Communications Systems, (Corrected Versionxe2x80x94Aug. 29, 1995).
A detailed sequence of activities are typically followed before the wireless unit can access the wireless communications system to establish a call to a phone number. As is known in the art, calls between a CDMA mobile station and a base station typically employ several kinds of channels. Initially, a pilot channel is employed as a reference signal for detecting other forward link channels. A sync channel is used to establish time and frame synchronization at the wireless unit. The sync channel message also provides information about another class of channels, the paging channels.
Paging channels are used to broadcast a variety of control information, including access channel information, contained in the access parameter message. This access parameter message contains information such as persistence parameters (0-9), persistence parameters (10-15), persistence modifiers MSG_PSIST and REG_PSIST, initial access power requirements, the number of access channels, the number of access attempts, maximum size of access messages, values for various overload classes, access attempt backoff parameters and other information of interest to mobiles seeking access to the base station.
To initiate a call and obtain access to the wireless communications system, the user inputs or dials the number into the wireless unit, and the wireless unit stores the phone number in storage, such as a memory. When the user finishes inputting the phone number, the user typically presses a send button to initiate the call. Before the wireless unit can access the wireless communications system, the wireless unit checks to ensure that the parameters received from the base station, such as those in the access parameter message, are current. After the wireless unit updates the parameters or determines that the parameters are current, the wireless unit commences the access procedure.
Access channel(s) are used by the wireless unit to obtain access to the wireless communications system, for example, to originate a call. The IS-95-A standard cited above provides a detailed sequence of activities that are to be followed in transmitting messages over an access channel to a base station. In systems based on the IS-95-A standard, CDMA wireless units transmit on the access channels according to a random access protocol in which the wireless unit attempts to gain access to the wireless communications system, referred to as an access attempt, by sending an access message and receiving (or failing to receive) an acknowledgment for that access message. As shown in FIG. 2, each transmission in the access attempt is called an access probe, and within an access attempt, access probes are grouped into access probe sequences. Each access attempt comprises up to max_req_seq (for a request access) or max_rsp_seq (for a response access) access probe sequences. Each access probe sequence consists of up to 1+NUM_STEP access probes, for example 15, all transmitted on the same Access Channel. The first access probe of each access probe sequence is transmitted at a specified power level relative to a nominal power level, and each subsequent access probe is transmitted at a power level a specified amount higher than the previous access probe.
The timing of access probes and access probe sequences is expressed in terms of Access Channel slots. The transmission of an access probe begins at the start of an Access Channel slot. As shown in FIG. 3, each such access probe comprises an access channel preamble and an access channel message capsule. The length of the preamble 1+pam_sz as well as the length of message capsule 3+max_cap_sz are expressed in terms of a number of 20 millisecond frames. Thus, the duration of an access probe (access channel slot) is 4+pam_sz+max_cap_sz frames.
FIGS. 4a and 4b show an example access procedure according to the IS-95-A standard. In attempting to gain access to the wireless communications system, two types of messages are typically sent on the access channel: a response message (one that is a response to a base station message), or a request message (one that is sent autonomously by the mobile). At block 30, counters PROBE and SEQ are initialized to count the access probe sequence numbers and the access probes. The transmission of an access probe begins at the start of an access channel slot. The precise timing of an access channel slot transmission in an access attempt is determined by a procedure called PN randomization. For the duration of each access attempt, the mobile station computes at block 32 a delay, RN, from 0 to 2probe-pn-ran PN chips using a (nonrandom) hash function that depends on its electronic serial number, ESN.
The access channel number, RA, used for each access probe sequence is chosen pseudo randomly from 0 to acc-chan among all the access channels associated with the current paging channel as shown in block 34. Once chosen, this access channel number is used by the wireless unit for all access probes in the current access probe sequence. The first access probe of each access probe sequence is transmitted at a specified power level relative to the nominal open loop power level as shown in block 36. The mobile station transmits the first probe in each access probe sequence at a mean output power level (referenced to the nominal CDMA channel bandwidth of 1.23 MHz) depending on open loop power estimate, the initial power offset for access init_pwr and the nominal transmit power offset nom_pwr.
The mobile station delays its transmit timing of each access probe by RN PN chips as shown in block 38. Timing between access probes of an access probe sequence is also generated pseudo-randomly. After transmitting each access probe, the mobile waits a specified period, TA=80xc3x97(2+acc_tmo) milliseconds from the end of the slot to receive an acknowledgment from the base station as shown by blocks 40 and 42. If an acknowledgment is received, the access attempt is successful as shown in block 44. If no acknowledgment is received within the time TA, the next access probe in the access probe sequence is transmitted after an additional backoff delay RT, from 1 to 1+probe_bkoff slots as shown by blocks 46-52. As shown in block 54, the next access probe and each subsequent access probe is transmitted at a power level a specified amount PI dB (determined from pwr_step) higher than the previous access probe until an acknowledgment response is obtained or the sequence ends as determined at block 48. Each access attempt comprises up to max_req_seq (for a request access) or max_rsp_seq (for a response access) access probe sequences as shown by blocks 56 and 58. If an acknowledgment has not been received after the last access probe sequence has been transmitted, the access attempt fails as shown in block 60. After an access probe sequence, a backoff delay, RS, of from 0 to 1+bkoff slots is generated pseudo randomly and applied as shown by blocks 62 and 64.
If the access attempt is an Access Channel request as determined at block 66, then before transmitting an access probe in each access probe sequence, the wireless station performs a persistence test 68 for every Access Channel slot. The wireless unit transmits the first access probe of an access probe sequence in a slot only if the wireless unit passes the persistence test for that slot. To perform the persistence test, the wireless unit generates a random number RP (0 less than RP less than 1) and compares it with a pre-determined threshold P as shown in blocks 70 and 72. If the generated random number RP is smaller than the pre-determined threshold P, transmission of the access probe sequence is initiated at step 34. If the persistence test fails, the access probe sequence is deferred until at least the next slot. Thus, in the case of access channel request, an additional delay PD can imposed before each access probe sequence by the persistence test. The pre-computed threshold P varies, depending on the nature of the request, the access overload class n, the persistence value psist(n) for the overload class which is found in the access parameters message from the base station, and its persistence modifier msg_psist (for message transmission) or reg_psist (or registrations). The maximum persistence value psist(n) is 63 for access overload classes 0 through 9, and is 7 for access overload classes 10 through 15. If the maximum persistence value is assigned to the wireless unit, then P=0 and the wireless unit ends the access attempt. For an Access Channel request by a wireless unit of access overload classes 0 through 9 (non-emergency), if psist(n) is not equal to 63, then, P is a monotonic decreasing function of psist(n) given the appropriate persistence modifier. For example, if the access channel request is a message request and psist (n) is not equal to 63, P is computed by P=2xe2x88x92psist(n)/4 * 2xe2x88x92msg-Psist. The larger the value for psist(n), the smaller the value for P, and the smaller the probability of initiating the access probe sequence. A larger value for P implies a higher probability of initiating the access probe sequence. For an Access Channel request by a wireless unit of access overload classes 10 through 15 (emergency), if psist(n) is not equal to 7, then, P is computed by 2xe2x88x92psist(n) * 2xe2x88x92msg-psist When in overload, the base station will try to increase the delay between access probes for ordinary overload classes of wireless units (e.g. class 0-9) rather than those for high priority and/or emergency classes (e.g. classes 10-15). In overload situations, P will decrease because the base station will increase the persistence values psist(n) for the ordinary overload classes, thereby making the persistence test even more difficult to pass. Thus, in an overload condition, the probability of an emergency class wireless unit initiating the access probe sequence is much higher than the non-emergency class wireless units.
Table 1 summarizes persistence test thresholds for various types of requests and access overload classes used in systems based on the IS-95-A standard. Table 1 shows that the maximum persistence value is 63 for access overload classes 0 through 9, and is 7 for access overload classes 10 through 15. If the maximum persistence value is assigned to the mobile station, the access attempts fails.
Table 2 summarizes average persistence delay for various types of access channel requests as known in systems based on the IS-95-A standard.
Thus, in an overload condition, a wireless unit in a normal overload class (e.g. overload class 0-9) can experience significant delay even when it is making a high priority call, such as an emergency call.
The present invention involves a high priority and/or emergency overload access control system in which a wireless unit is treated as a higher priority wireless unit when the wireless unit is attempting to access a wireless communications system with a high priority call, such as an emergency call. For example, the overload access control system can recognize if the wireless unit is attempting to initiate a call to an emergency number. If so, the wireless unit can perform a persistence test as an emergency class (e.g. overload class 10-15) wireless unit in attempting to access the wireless communications system. Thus, the wireless unit will experience an increased probability of passing the persistence test and thereby reducing the persistence delay in attempting to access the wireless communications system with the emergency call.